UNIT -3 1. SOLID GROUND CURING 2 Highlights: 1. Large parts of 500x500x350mm can be fabricated quickly. 2. High speed allows production of many parts.

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SOLID GROUND SOLID GROUND CURINGCURING

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Highlights:Highlights:

1. Large parts of 500x500x350mm can 1. Large parts of 500x500x350mm can be fabricated quickly.be fabricated quickly.

2. High speed allows production of many 2. High speed allows production of many parts.parts.

3. Masks are created.3. Masks are created. 4. No post curing required.4. No post curing required.

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5. Milling step ensures flatness of 5. Milling step ensures flatness of subsequent layers.subsequent layers.

6. Wax supports model, hence no extra 6. Wax supports model, hence no extra support is required.support is required.

7. Create a lot of wastes.7. Create a lot of wastes. 8. Not as prevalent as SLA and SLS but 8. Not as prevalent as SLA and SLS but

gaining ground because of high throughput gaining ground because of high throughput and large parts.and large parts.

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Process:Process: The steps in process are as follows The steps in process are as follows First a CAD model of the part is created First a CAD model of the part is created

and it is sliced in to layers using and it is sliced in to layers using cubitos cubitos data front end softwaredata front end software..

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1. Spray photosensitive resin:At the beginning of a layer creation step the flat work surface is sprayed with photosensitive resin.

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2. Development of photomaskFor each layer a photomask is produced using cubitals proprietary ionographic printing technique.

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3. Expose photomaskThe photomask is positioned over the work surface a powerful UV lamp hardens the exposed photosensitive resin.

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4. Vacuum uncured resin and solidify the remnantsAfter the layer is cured all the uncured resin is vacuumed for recycling leaving the hardened area intact the cured layer is passed beneath a strong linear UV lamp to fully cure in and solidify any remnants particles as shown in figure.

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5. Wax is applied to replace uncured resin areaWax replaces the cavities left by vacuuming the liquid resin. The wax is hardened by cooling to provide continuous solid support for the model as it is fabricated extra supports are not needed.

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6. The top surface is milled flatIn the final step before the next layer, the wax resin surface is milled flat to an accurate reliable finish for next layer.

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Once all layers are completed the wax is removed and any finishing operations such as sanding etc can be performed no post curing is necessary.

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AdvantagesAdvantages The entire layer is solidified at once.The entire layer is solidified at once. Reduction in the part build time for Reduction in the part build time for

multipart builds.multipart builds. Larger prototypes can be nested to utilize Larger prototypes can be nested to utilize

the build volume fully.the build volume fully. No postcuring is required.No postcuring is required.

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DisadvantagesDisadvantages The system is large, noisy and heavy.The system is large, noisy and heavy. It wastes a large amount of wax which It wastes a large amount of wax which

cannot be recycled.cannot be recycled. SGC systems are prone to breakdowns.SGC systems are prone to breakdowns. The resin models of SGC are not suitable The resin models of SGC are not suitable

for investment casting because coefficient for investment casting because coefficient of thermal expansion is more than ceramics of thermal expansion is more than ceramics in resin which may lead to cracks in casting.in resin which may lead to cracks in casting.

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Laminated object Laminated object ManufacturingManufacturing

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IntroductionIntroduction

Laminated Object Manufacturing is a Laminated Object Manufacturing is a rapid prototyping technique that produces rapid prototyping technique that produces 3D models with paper, plastics or 3D models with paper, plastics or composites. composites.

LOM was developed by Helices LOM was developed by Helices Corporation, Torrance, California. LOM is Corporation, Torrance, California. LOM is actually more of a actually more of a hybrid between hybrid between subtractive and additive processsubtractive and additive process..

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In this technique models are built up with In this technique models are built up with layers of cross section of the part. Hence as layers of cross section of the part. Hence as layers are been added, the excess material layers are been added, the excess material is not required for that cross section is being is not required for that cross section is being cut away.cut away.

LOM is one of the fastest RP processes for LOM is one of the fastest RP processes for parts with longer cross sectional areas which parts with longer cross sectional areas which make it ideal for producing large parts.make it ideal for producing large parts.

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System Hardware:System Hardware: LOM system is available in two sizes. LOM system is available in two sizes. LOM 1015 produces parts up to 10x15x14 inches.LOM 1015 produces parts up to 10x15x14 inches. LOM 2030 produces parts upto20x30x24 inches.LOM 2030 produces parts upto20x30x24 inches. Common build material is paper.Common build material is paper. Build material has pressure and heat sensitive Build material has pressure and heat sensitive

additive on the banking.additive on the banking. Material thickness ranges from 0.0038-0.005 Material thickness ranges from 0.0038-0.005

inches.inches.

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Softwares:Softwares:LOM SLICE SOFTWARE:LOM SLICE SOFTWARE:

It provides interface between operator It provides interface between operator and the system. LOM does not require a and the system. LOM does not require a pre slice of STL FILE i.e. once the pre slice of STL FILE i.e. once the parameters are loaded into LOM SLICE, parameters are loaded into LOM SLICE, the STL file slices as the part builds. The the STL file slices as the part builds. The process of continuous slicing is called process of continuous slicing is called slice on the flyslice on the fly..

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The LOM has a The LOM has a feed spindle feed spindle and a and a take up take up spindlespindle for the build material. for the build material.

The feed spindle holds the roll of virgin The feed spindle holds the roll of virgin material whereas the take up spindle serves material whereas the take up spindle serves to to store the excess material after the store the excess material after the layer is cut.layer is cut.

A A heated roller heated roller travels across the face of travels across the face of the part being built after each layer to the part being built after each layer to activate adhesive and bond the part layer activate adhesive and bond the part layer together.together.

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An invisible An invisible 25Watts CO2 laser 25Watts CO2 laser is housed is housed on the back of the LOM and reflected off on the back of the LOM and reflected off three mirrors before finally passing through a three mirrors before finally passing through a focusing lens on the carriage. focusing lens on the carriage.

The The carriage moves in the X direction carriage moves in the X direction and and the the lens moves in the Y direction lens moves in the Y direction on the on the carriage, thus allowing focal cutting point of carriage, thus allowing focal cutting point of laser to be moved like a plotter pen while laser to be moved like a plotter pen while cutting through build material in the shape cutting through build material in the shape desired.desired.

This X and Y movement allows for two This X and Y movement allows for two degrees of freedom or essentially a degrees of freedom or essentially a 2-D 2-D sketch of part cross sectionsketch of part cross section. The part being . The part being built is adhered to a removable metal plate built is adhered to a removable metal plate which holds the part stationary until it is which holds the part stationary until it is completed. completed.

The The plate is bolted to the platen with plate is bolted to the platen with bracketsbrackets and moves in the Z direction by and moves in the Z direction by means of a large threaded shaft to allow the means of a large threaded shaft to allow the parts to be built up. This provides the third parts to be built up. This provides the third degree of freedom where in the LOM is able degree of freedom where in the LOM is able to build 3D models.to build 3D models.

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Some Some smoke and other vapors are smoke and other vapors are created created since the LOM functions by since the LOM functions by essentially burning through the sheets of essentially burning through the sheets of material with a laser, therefore LOM must material with a laser, therefore LOM must be ventilated either to the outside air or be ventilated either to the outside air or through a large filtering device at rates through a large filtering device at rates around 500cubic feet per minute.around 500cubic feet per minute.

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LOM OPERATION:LOM OPERATION:

The way the LOM constructs the parts is The way the LOM constructs the parts is by by consecutively adhering layers of consecutively adhering layers of build material while cutting the cross build material while cutting the cross section of the parts with a lasersection of the parts with a laser. The . The LOM SLICE software that comes with LOM SLICE software that comes with LOM machine controls all these. The LOM machine controls all these. The following description of operation is following description of operation is described with paper as build material.described with paper as build material.

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SOFTWARE:SOFTWARE:

As with all RP systems, the LOM must As with all RP systems, the LOM must begin with the standard RP computer file begin with the standard RP computer file or STL file.or STL file.

The STL is loaded into the LOM SLICE The STL is loaded into the LOM SLICE which graphically represents the model which graphically represents the model on the screen.on the screen.

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Upon loading the STL file, LOM SLICE Upon loading the STL file, LOM SLICE creates creates initialization filesinitialization files in the in the background for controlling the LOM background for controlling the LOM machine. Now there are machine. Now there are several several parametersparameters the user must consider and the user must consider and enter before building the part.enter before building the part.

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Part Orientation:Part Orientation:

The designed shape of the parts to be The designed shape of the parts to be built in LOM must be evaluated for built in LOM must be evaluated for determining the orientation in which to determining the orientation in which to build the parts.build the parts.

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First Consideration:First Consideration:

Accuracy desired for curved surfaces: Accuracy desired for curved surfaces: Parts with curved surfaces tend to have a Parts with curved surfaces tend to have a better finish if the curvatures of the cross better finish if the curvatures of the cross sections are cut in the sections are cut in the XY planeXY plane. This is . This is true due to the fact that the controlled true due to the fact that the controlled motion of the laser cutting in the XY plane motion of the laser cutting in the XY plane can hold better curve tolerances can hold better curve tolerances dimensionally than the layered effects of dimensionally than the layered effects of XZ and YZ planes. XZ and YZ planes.

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If a part contains If a part contains curvatures in more curvatures in more than one planethan one plane, one alternative is to , one alternative is to build the part at an build the part at an angle to the axisangle to the axis. . The benefits here are too full as the part The benefits here are too full as the part will not only have will not only have more accurate more accurate curvatures curvatures but will also tend to have but will also tend to have better laminar strength better laminar strength across the across the length of the part.length of the part.

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Second Consideration:Second Consideration:

Time taken to fabricate a part: Time taken to fabricate a part: The The slowest aspect of build process for LOM is slowest aspect of build process for LOM is movement in Z direction or time between the movement in Z direction or time between the layers. This is mainly because after laser layers. This is mainly because after laser cuts across the surface of the beam cuts across the surface of the beam material, the LOM must bring more paper material, the LOM must bring more paper across the top face of the part and then across the top face of the part and then adhere to the previous layer before the laser adhere to the previous layer before the laser can begin cutting again.can begin cutting again.

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For this reason a general rule have come For this reason a general rule have come for orienting long narrow parts is to place for orienting long narrow parts is to place the the lengthiest sections in the XY lengthiest sections in the XY planeplane. This way the slowest part of the . This way the slowest part of the process the actual laser cutting is process the actual laser cutting is minimized to a smaller amount of layers.minimized to a smaller amount of layers.

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There are some third party software There are some third party software renders that have automatic testing renders that have automatic testing functions that will strategically place parts functions that will strategically place parts in optimum orientations for the selected in optimum orientations for the selected section.section.

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Cross Hatching:Cross Hatching:

Cross Hatching is necessary to get rid of Cross Hatching is necessary to get rid of excess paper on the individual layers. excess paper on the individual layers. Cross hatch sizes are set in LOM SLICE Cross hatch sizes are set in LOM SLICE by the operator and can vary throughout by the operator and can vary throughout the part. the part.

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Basically the operator puts in a range of layers for which we want a certain cross hatch pattern for sections of the part that do not have integrate features or cavities, a larger cross hatch can be set to make a part build faster but for thin walled sections and hollowed out areas, a finer cross hatch will be easier to remove.

The cross hatch size is given in values of X and Y. Therefore the hatch pattern can vary from square to long thin rectangles.

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The two main considerations for cross hatching The two main considerations for cross hatching areare

Ease of part removal.Ease of part removal. Resulting build time.Resulting build time. A very small hatch sizes will make for easy part A very small hatch sizes will make for easy part

removal. However if the part is rather large or removal. However if the part is rather large or has large void areas it can really slow down the has large void areas it can really slow down the build time. This is the reason for having varying build time. This is the reason for having varying cross hatch sizes throughout the part.cross hatch sizes throughout the part.

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The LOM The LOM operator can either judgeoperator can either judge where where and how the part should be cross hatched and how the part should be cross hatched visually or use LOM slice to run a visually or use LOM slice to run a simulationsimulation build on the computer screen to determine build on the computer screen to determine layer ranges for the needed hatch sizes.layer ranges for the needed hatch sizes.

Also since the LOM SLICE creates slices as Also since the LOM SLICE creates slices as the part build parameters can be changed the part build parameters can be changed during a build simply by during a build simply by pausingpausing a LOM a LOM machine and typing in new cross hatch machine and typing in new cross hatch values.values.

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System Parameters:System Parameters:

There are various controlling parameters There are various controlling parameters such as laser power, heater speed, such as laser power, heater speed, material advance margin, and support material advance margin, and support wall thickness and heater compression.wall thickness and heater compression.

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Laser Power: Laser Power: It is the percentage of total It is the percentage of total laser output wattage.laser output wattage.

For e.g. LOM 1015 is operated at a laser For e.g. LOM 1015 is operated at a laser power of about 9% of maximum 25W power of about 9% of maximum 25W laser or approximately 2.25W. This value laser or approximately 2.25W. This value will be different for various materials or will be different for various materials or machines but essentially it is set to cut machines but essentially it is set to cut through only one sheet of build material.through only one sheet of build material.

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Heater Speed:Heater Speed: It is the rate at which hot It is the rate at which hot roller passes across the top of the part. roller passes across the top of the part. The rate is given in inches/second. It is The rate is given in inches/second. It is usually 6”/sec for `initial pass and 3”/sec usually 6”/sec for `initial pass and 3”/sec for returning pass of heater. The heater for returning pass of heater. The heater speed effects the lamination of the sheet speed effects the lamination of the sheet so it must be set low enough to get a so it must be set low enough to get a good bond between layers.good bond between layers.

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Material Advance Margin: Material Advance Margin: It is the It is the distance the paper is advanced in addition distance the paper is advanced in addition to length of the part.to length of the part.

Support Wall Thickness:Support Wall Thickness: It controls the It controls the outer support box walls throughout a part. outer support box walls throughout a part. The support wall thickness is generally set The support wall thickness is generally set 0.25” in the X and Y direction, although this 0.25” in the X and Y direction, although this value can be changed by operator.value can be changed by operator.

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Compression:Compression: It is used to set the It is used to set the pressure that the heater roller exerts on pressure that the heater roller exerts on the layer. It is measured in inches which the layer. It is measured in inches which are basically the distance the roller is are basically the distance the roller is lifted from its initial track by the top lifted from its initial track by the top surface of part. Values for compression surface of part. Values for compression will vary for different machines and will vary for different machines and materials, but are typically 0.015”-0.025”.materials, but are typically 0.015”-0.025”.

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Advantages of LOM:Advantages of LOM:

1. Ability to produce large scale models using 1. Ability to produce large scale models using very inexpensive paper material.very inexpensive paper material.

2. The 2. The finishing abilityfinishing ability of parts and of parts and good good handling strengthhandling strength couples with couples with speed and speed and accuracyaccuracy to provide an all round quality to provide an all round quality models.models.

3. Material are 3. Material are environmentally compliant environmentally compliant and have not shown any capability of being and have not shown any capability of being health threatening.health threatening.

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Disadvantages of LOM:Disadvantages of LOM:

1. Need for decubing.1. Need for decubing. 2. Emission of smoke fumes can be slight 2. Emission of smoke fumes can be slight

nuisance.nuisance. 3. Machining operations by bonding 3. Machining operations by bonding

through paper can raise some concern to through paper can raise some concern to fire safety officials.fire safety officials.

4. Large amount of scrap is being 4. Large amount of scrap is being generated in the process.generated in the process.

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Uses of LOM:Uses of LOM:

1. Concept Verification.1. Concept Verification. 2. Fit Check Analysis.2. Fit Check Analysis. 3. Direct use components.3. Direct use components. 4. Casting and moulding patterns.4. Casting and moulding patterns.

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